1. Field of the Invention
The present invention relates to a power semiconductor device including plural power semiconductor elements to drive a load.
2. Description of the Background Art
A power semiconductor element such as a power MOSFET (Metal Oxide Semiconductor Field Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) and so on is applied as a control device to drive a load flowing a large current.
A technique of a power semiconductor device that such the power semiconductor element is plurally connected in parallel and a controllable current amount is made to increase is described in Japanese Patent Application Laid-Open No. 8-191239 (1996). Besides, Japanese Patent Application Laid-Open Nos. 2000-92820, 2002-95240, 2001-169401 and 2002-208849 are additional prior art documents corresponding to the present invention.
In case that the plural power semiconductor elements are connected in parallel, a current tends to flow in a particular element in the parallel connection in larger amounts, if there is a difference in an electric characteristic of the respective elements. According to this, an element in which the current flows in larger amounts runs a temperature substantially and a life of the element tends to be short.
In the case that the plural power semiconductor elements are connected in parallel, a current tends to flow in a particular element in the parallel connection in larger amounts, if there is a difference in an electric characteristic of the respective elements. According to this, an element in which the current flows in larger amounts runs a temperature substantially higher and a life of the element tends to be short.
Moreover, there is a steady loss occurring in an ON action and a switching loss occurring in a switching period in a power loss generated in the power semiconductor element. The steady loss is attributable mainly to an ON resistance in the ON action, and the switching loss is attributable mainly to a tail current in a turn-off state.
The steady loss increases according to an increase of a conducting current of the power semiconductor element. In the meantime, a value of the tail current which is a major reason for the switching loss is substantially constant regardless of the conducting current amount. Accordingly, the amount of the power loss caused by the tail current is proportional to the number of the parallel connection of the power semiconductor element and the number of switching of the power semiconductor element.
That is to say, the more the number of the parallel connection of the power semiconductor element increases and the higher a switching frequency becomes, the more the amount of the power loss caused by the tail current is supposed to increase. Particularly, in case of bringing into action with a current value low enough as compared to a rated current of the power semiconductor element and with a high switching frequency, proportion of the switching loss to an overall loss which is an aggregation of the steady loss and the switching loss becomes high, thus the amount of the power loss caused by the tail current has a great influence.